In the treatment of malignant tumors, the success rate for removing of primary cancers has risen steadily due to advances in surgery, radiation and chemotherapy. However, even in the early stages of a tumor, malignant tumor cells often scatter and invade locally into adjacent tissues and organs. In this case, even if the primary focus of malignant tumor is removed surgically, recurrence is possible due to residual invading malignant tumor cells; ultimately resulting in death in many cases. In particular, if malignant tumor cells have invaded into a vital organ, they cannot be removed along with normal tissues. So the recurrence rate and death rate are high in these cases. When a malignant tumor occurs in a vital organ, large amounts of tissue including normal tissues are removed in an effort to avoid local invasion, and the function of the vital organ may be lost. Moreover, in the case of malignant tumors with a high degree of malignancy such as melanoma, lung cancer, liver cancer and pancreatic cancer, early diagnosis is difficult. By the time the tumor is diagnosed, the local invasion of malignant tumor has become widespread, making unacceptable for surgical treatment in many cases.
Radiation usually does not demonstrate positive results for treating the local invasion of these malignant tumors. Moreover, the chemotherapy drugs currently in clinical use, such as adriamycin, which act by directly attacking malignant tumor cells, also attack normal cells, and have strong side-effects; a problem for clinical use. Consequently, new preparations for inhibiting local invasion of malignant tumors are expected.
It is generally believed that local invasion is related as follows to the stages of malignant tumors.
(1) In the early stage of malignant tumors, malignant tumor cells proliferate by cell division, resulting in the tumor tissue's growth. This is known as “dysplasia”. In histopathological terms, the proliferating malignant tumor cells are localized at this stage, and can be clearly distinguished from the surrounding normal tissues. In most cases, capsule-like tissue forms around the malignant tumor tissues.
Capsule-like tissue is connective tissue formed as a result of the interaction between malignant tumor tissues and normal tissues, separating the malignant tumor tissues from the normal tissues. So it appears that the malignant tumor tissues are surrounded by a capsule. A malignant tumor enveloped by capsule-like tissue is extremely easy to remove by surgery, and the malignant tumor cells are not scattered during the surgery, there is very little risk of the malignant tumor recurring after surgery.
(2) However, as the malignant tumor progresses, the capsule-like tissue is lost, and there is local invasion from the malignant tumor tissues to adjacent surrounding normal tissues and organs, resulting in wider diffusion of the malignant tumor. At this stage, surgical treatment becomes difficult.
Considering the aforementioned relationship between local invasion and the stages of malignant tumors, it has been suggested that malignant tumor therapies that acted by promoting the formation of capsule-like tissue, might be clinically useful. However, no studies of capsule-like tissue formation around malignant tumors have been done focusing solely on local invasion of malignant tumors, and no drugs have been developed that promote such formation (see, for example, Clin. Expl. Metastasis 7:277-282, 1989). This is because in the technical field, local invasion of malignant tumors is generally considered to be a part of malignant tumor metastatic process, and it is believed that drugs that inhibit metastasis should also be able to inhibit local invasion.
Recently, however, pathway of metastasis which does not involve a local invasion process has been discovered in many malignant tumors (see, for example, BMC Medicine, 2(9):1-8, 2004). For example, there have been clinical reports of malignant tumors that metastasize without local invasion (see, for example, Virchows Arch. Pathol. Anat. 390:121-126, 1981 and Surgery Today 25:369-372, 1995). Moreover, drugs that have been reported to be effective in inhibiting local invasion of malignant tumors but are ineffective against metastasis and, in fact, promote metastasis (see, for example, Breast Cancer Res. Treat. 40:209-223, 1996). Conversely, there have been reports of in vivo experiments with drugs that inhibit malignant tumor metastasis but do not inhibit local invasion (see, for example, Clinical & Experimental Metastasis, 19:95-105, 2002). These reports suggest a phenomenon in which local invasion of malignant tumors is not a part of the metastatic process, but instead, that local invasion and metastasis occur as different processes.
From another perspective, much basic and clinical researches have pointed to a close relationship between malignant tumors and the coagulation and fibrinolytic system. For example, microcirculation disorders caused by increased plasma fibrinogen concentration, increased blood viscosity, abnormal blood rheology and other abnormalities of the coagulation and fibrinolytic system, are known to occur in malignant tumor patients. It has also been reported that increased plasma fibrinogen concentration or secretion of fibrinogen by the malignant tumor cells themselves leads to the deposition of fibrinogen or fibrin in the extracellular matrix of the malignant tumor tissue, and that this then acts as part of the extracellular matrix to promote proliferation, invasion and metastasis of the malignant tumor cells (see, for example, Cancer Research 60:2033-2039, 2000; Ann NY Acad. Sci. 936:406-425, 2001 and Blood 96:3302-3309, 2000).
Focusing on this relationship between malignant tumors and the coagulation and fibrinolytic system, it has been reported that batroxobin, a thrombin-like serine protease from Bothrops atrox moojeni venom, inhibits proliferation and metastasis of malignant tumors in the same way as Ancrod, another thrombin-like serine protease (see, for example, Eur. J. Cancer 16:919-923, 1980). However, while this report uses tumor weight as an indicator of the inhibitive effect on malignant tumor proliferation, and the number of metastatic foci in organs as an indicator of malignant tumor metastasis, it does not evaluate the local invasion of malignant tumors.
Also in connection with the relationship between malignant tumors and the coagulation and fibrinolytic system, it has been reported that in an experimental model using fibrinogen-deficient mice, malignant tumor metastasis is dependent on fibrinogen and fibrin, but malignant tumor proliferation and local invasion are not dependent on fibrinogen (see, for example, Blood 96:3302-3309, 2000). However, the relationship between batroxobin and local invasion of malignant tumors has not been reported.